Conventional diaphragm-type carburetors with a diaphragm fuel pump can produce increased vibration in a fuel metering chamber of the carburetor. This, along with engine vibration of the carburetor, and the heat transferred to the fuel in the carburetor, tends to generate increased fuel vapor in the fuel metering chamber. If the vapor is not removed satisfactorily it can block or severely restrict fuel flow through a fuel jet to bring forth a so-called vapor lock state, wherein the supply of fuel to the engine is at least temporarily interrupted resulting in unsteady engine operation or an engine stall. Japanese Patent Publication No. 28341/1968 and Japanese patent Laid-Open No. 131807/1998 disclose techniques to alleviate this problem. However, vapor lock is still possible under severe engine load conditions.